1. Field of the Invention
This invention relates to a silicone resin-containing emulsion composition having film-forming ability and providing cured films having improved properties including mar resistance, weather resistance and chemical resistance, a method for preparing the same, and an article having a cured film of the composition.
2. Prior Art
In the paint and coating field, a change of dispersing media from organic solvents to aqueous systems is desired from the standpoints of environmental protection and safe working environment.
To meet such demands, emulsion paints obtained by emulsion polymerizing radical-polymerizable vinyl monomers as typified by acrylic resin systems are widely employed as the base of coating compositions because of their good film-forming ability and chemical resistance. However, coating compositions of this type have the drawback that they essentially lack water resistance and weather resistance.
On the other hand, silicone resins resulting from hydrolytic condensation of silane compounds have found acceptance as the coating agent since they are able to form films having a high hardness and satisfactory weather resistance, water resistance, heat resistance and water repellency. However, they have the drawbacks that film-forming ability, alkali resistance, and film flexibility are poor, and shelf stability is also poor due to the high condensation activity of silanol groups. Also, the silicone resins are generally used in the form of solutions in organic solvents, which have such problems as the hazard of fire or explosion, toxicity to humans, and environmental contamination. It is thus desired to have silicone resin compositions of the emulsion type.
To overcome the above and other drawbacks, a number of attempts have been made as described below although they are still unsatisfactory.
(a) JP-A 213046/1983, 197369/1987, 115485/1991, and 200793/1991 disclose a method of emulsifying alkoxysilane compounds or partial hydrolytic condensates thereof with the aid of surfactants to form aqueous emulsions. It is also known to mix this emulsion with an emulsion resulting from emulsion polymerization of a polymerizable vinyl monomer (JP-A 344665/1994).
(b) JP-A 247434/1995 discloses a method of forcedly mechanically grinding and dispersing a solventless solid silicone resin along with a chain silicone compound to form an aqueous emulsion.
(c) JP-A 60098/1996 discloses a method of emulsion polymerizing a radical-polymerizable vinyl monomer in the presence of an aqueous polymer resulting from hydrolysis of an alkoxysilane in water without using a surfactant.
(d) JP-A 209149/1993 and 196750/1995 disclose a method comprising hydrolyzing and condensing an alkoxysilane mixture containing a vinyl-polymerizable alkoxysilane to form an aqueous emulsion containing a solid silicone resin, adding a radical-polymerizable vinyl monomer thereto, and effecting emulsion polymerization, thereby forming a graft copolymer fine particle (solid) emulsion.
(e) JP-A 45628/1991 and 3409/1996 disclose a method for introducing a silicone resin into emulsion particles by adding an alkoxysilane to an emulsion resulting from emulsion polymerization of a radical-polymerizable vinyl monomer, and effecting hydrolytic condensation.
(f) JP-A 9463/1986 and 27347/1996 disclose a method of emulsion polymerizing a vinyl-polymerizable functional group-containing alkoxysilane with a radical-polymerizable vinyl monomer to form an emulsion.
However, method (a) has the drawbacks that active alkoxy groups are susceptible to hydrolysis with the lapse of time to form an alcohol, which is an organic solvent, as a by-product in the system, and a relatively low polymeric raw material converts to a higher molecular weight one. As a result, the alcohol by-product renders the emulsion unstable, and the characteristics are variable because of a changing degree of polymerization. These drawbacks can be compensated for by using a large amount of surfactant, but at the sacrifice of film properties including hardness and water resistance. Additionally, since the system is based on a silicone resin alone, the drawbacks inherent to the silicone resin are maintained and film properties are not fully satisfactory.
In method (b), particles of a solid silicone resin having a softening point are forcedly emulsified along with a chain organopolysiloxane having an OH group at each end with the aid of a surfactant. This method has the advantage that no organic solvents are contained, but the drawback that the solid silicone resin, because of its high polarity, is difficultly dissolvable in low polar liquid organopolysiloxane. Since this requires to use a sand grinder in dispersing the solid resin, it is difficult to reduce the size of particles and therefore, the resulting emulsion remains less stable. Because of the solid resin, the content of silanol groups is as low as 0.1 to 5% by weight, which requires high temperatures of 200 to 300.degree. C. for curing. Then the working efficiency is lower than the organic solvent type. Because of the combined use of chain organopolysiloxane, the cured film is fully water repellent, but is low in hardness and thus unsatisfactory in mar resistance and durability.
Method (c) starts with simple hydrolysis of an alkoxysilane in water to form a water-soluble resin free of a surfactant. A surfactant is then added thereto, after which a radical-polymerizable vinyl monomer is emulsion polymerized. However, this method also has the disadvantage that hydrolysis in water is accompanied by formation of an alcohol by-product. The water-soluble silicone resin is partially incorporated in the emulsion particles, but the majority is left in water because of a high water solubility. Highly active silanol groups contained in the molecules in water gradually undergo condensation, precipitating as a gel-like matter. As a consequence, the emulsion tends to become unstable. Since the silicone resin component and the vinyl polymerizable component are essentially incompatible with each other and the respective components are not uniformly distributed within the cured film, film properties become poor. Method (c) is not fully satisfactory.
In method (d), alkoxysilanes are hydrolyzed in water to form an emulsion of a corresponding solid silicone resin. Utilizing vinyl polymerizable groups in the silicone resin as starting points, acrylic chains are then grafted by emulsion polymerization. Since this method also involves the hydrolysis step of alkoxysilanes, the system contains an alcohol which is an organic solvent. Therefore, the emulsion remains relatively unstable. Aiming to provide solid fine particles capable of imparting lubricity and water repellency, this method is to prepare composite particles in which the silicone resin serving for that function is located at the center of the core and acrylic chains contributing to anti-agglomeration of particles in water and improved dispersion are grafted thereto. Accordingly, this method favors that both the components are present in a non-uniform state, and that the silicone resin undergoes full crosslinking within the particles and rather, contains no silanol groups and is thus inactive. No satisfactory cured films are obtained.
Since an alkoxysilane is post added to an emulsion in method (e), like the other methods, method (e) has the drawback that an alcohol by-product is formed and the complete incorporation of the silicone resin in emulsion particles is difficult. The stability of the emulsion and the properties of cured film are below the satisfactory level. Because of the post-addition, there is a high possibility that a greater proportion of the silicone resin is present in the shell. Therefore, both the components within particles and cured film are non-uniform and the compensation of properties by both the resins is insufficient.
In method (f), a vinyl-polymerizable functional group-containing alkoxysilane is emulsion polymerized with a radical-polymerizable vinyl monomer. Since the alkoxy groups are restrained of hydrolysis and thus retained, this emulsion eliminates the formation of an alcohol by-product as in the other methods and minimizes the change of properties with time. However, it is difficult to introduce a large proportion of the silicone resin component into the film, and therefore, such properties as weather resistance are least improved and unsatisfactory when the exterior application is considered.
As described above, the prior art known methods fail to provide a silicone resin-containing emulsion which (i) does not contain flammable low-boiling organic solvents such as alcohols, (ii) remains highly stable, (iii) is low-temperature curable, and (iv) forms a uniform cured film having improved properties including mar resistance, weather resistance, and chemical resistance.
Therefore, an object of the present invention is to provide a silicone resin-containing emulsion composition satisfying the above requirements (i) to (iv), a method for preparing the same, and an article having a cured film of the composition.